Multiplatform Physiologic and Metabolic Phenotyping Reveals Microbial Toxicity

The microbiome- and metabolome-modulating activity of dietary cholesterol: insights from the small and large intestines

Cholesterol is an important lipid molecule that affects the gut microbiome upon ingestion. We systematically investigated the effects of cholesterol on the microbiota of the large and small intestines using ex vivo and in vivo models, combining flow cytometry, metabolomics, and metagenomics. The results showed that cholesterol directly causes a loss of bacterial membrane polarity and integrity, as well as a reduction in microbial metabolic activity. Cholesterol directly affected the global metabolism of the large and small intestinal microbiota, including amino acid, carbohydrate, and nucleotide metabolism. Ex vivo and in vivo studies shared similar results, showing that cholesterol increased the abundance of the primary bile acid-metabolizing bacteria Clostridium and Dorea in the large intestinal microbiota, confirming the enrichment effect of cholesterol on these bacteria. In the in vivo model, increased conjugated bile acids in the small intestine and decreased abundance of BSH-containing Bifidobacterium were observed due to cholesterol. Only in vivo models have demonstrated that cholesterol increases phosphatidylcholine levels in both the small and large intestines, which may be related to the effects of cholesterol on host metabolism. The pro-inflammatory capacity of the intestinal microbiota was enhanced by cholesterol, as evidenced by the increased levels of IL-1β and TNF-α in THP-1 cells upon stimulation with cholesterol-treated microbiota. This study comprehensively elucidates the effects of cholesterol on the composition and metabolic functions of the microbiota in both the large and small intestines. It offers a novel perspective on the ways in which cholesterol affects host metabolism via the gut microbiome.

Chlorpyrifos modulates the mouse gut microbiota and metabolic activity

The organophosphate chlorpyrifos is a commonly used pesticide for fruits and vegetables despite its association with neurotoxicity in humans. While some studies suggest that organophosphates may impact the gut microbiota, no studies to date have investigated the direct effect of chlorpyrifos on the gut microbiota with doses that approximate environmentally relevant dietary concentrations (EPA chronic reference dose: 0.3 µg/kg/day in humans and EPA acute reference dose: 5 µg/kg/day in humans). Thus, we examined the influence of chlorpyrifos on the gut microbiota by assessment of bacterial physiology and metabolism using flow cytometry, 1H NMR-based metabolomics, and changes in the cecal microbiota community with 16S rRNA amplicon sequencing and analysis. Chlorpyrifos did not directly damage bacteria but rather perturbed bacterial metabolism. Chlorpyrifos exposure to bacteria increased the concentration of amino acids, carbohydrates, and nucleic acids. The relative abundances of Lactobacillus, Allobaculum, Roseburia, and Butyricicoccus increased after exposure to chlorpyrifos. Analyses of the 16S rRNA gene amplicon data predicted decreased amino acid biosynthesis and nucleic acid degradation and increased glycolysis which was supported by 1H NMR-based metabolomics. Collectively, these results demonstrate that environmentally relevant doses of chlorpyrifos can impact the metabolic activity of isolated gut microbes which may result in an imbalance in overall gut metabolic activity.

Effects of Early Life Exposures to the Aryl Hydrocarbon Receptor Ligand TCDF on Gut Microbiota and Host Metabolic Homeostasis in C57BL/6J Mice

BACKGROUND

Exposure to persistent organic pollutants (POPs) and disruptions in the gastrointestinal microbiota have been positively correlated with a predisposition to factors such as obesity, metabolic syndrome, and type 2 diabetes; however, it is unclear how the microbiome contributes to this relationship.

OBJECTIVE

This study aimed to explore the association between early life exposure to a potent aryl hydrocarbon receptor (AHR) agonist and persistent disruptions in the microbiota, leading to impaired metabolic homeostasis later in life.

METHODS

This study used metagenomics, nuclear magnetic resonance (NMR)- and mass spectrometry (MS)-based metabolomics, and biochemical assays to analyze the gut microbiome composition and function, as well as the physiological and metabolic effects of early life exposure to 2,3,7,8-tetrachlorodibenzofuran (TCDF) in conventional, germ-free (GF), and Ahr-null mice. The impact of TCDF on Akkermansia muciniphila (A. muciniphila) in vitro was assessed using optical density (OD 600), flow cytometry, transcriptomics, and MS-based metabolomics.

RESULTS

TCDF-exposed mice exhibited lower abundances of A. muciniphila, lower levels of cecal short-chain fatty acids (SCFAs) and indole-3-lactic acid (ILA), as well as lower levels of the gut hormones glucagon-like peptide 1 (GLP-1) and peptide YY (PYY), findings suggestive of disruption in the gut microbiome community structure and function. Importantly, microbial and metabolic phenotypes associated with early life POP exposure were transferable to GF recipients in the absence of POP carry-over. In addition, AHR-independent interactions between POPs and the microbiota were observed, and they were significantly associated with growth, physiology, gene expression, and metabolic activity outcomes of A. muciniphila, supporting suppressed activity along the ILA pathway.

CONCLUSIONS

These data obtained in a mouse model point to the complex effects of POPs on the host and microbiota, providing strong evidence that early life, short-term, and self-limiting POP exposure can adversely impact the microbiome, with effects persisting into later life with associated health implications. https://doi.org/10.1289/EHP13356.

Bile acid metabolism and signaling, the microbiota, and metabolic disease

The diversity, composition, and function of the bacterial community inhabiting the human gastrointestinal tract contributes to host health through its role in producing energy or signaling molecules that regulate metabolic and immunologic functions. Bile acids are potent metabolic and immune signaling molecules synthesized from cholesterol in the liver and then transported to the intestine where they can undergo metabolism by gut bacteria. The combination of host- and microbiota-derived enzymatic activities contribute to the composition of the bile acid pool and thus there can be great diversity in bile acid composition that depends in part on the differences in the gut bacteria species. Bile acids can profoundly impact host metabolic and immunological functions by activating different bile acid receptors to regulate signaling pathways that control a broad range of complex symbiotic metabolic networks, including glucose, lipid, steroid and xenobiotic metabolism, and modulation of energy homeostasis. Disruption of bile acid signaling due to perturbation of the gut microbiota or dysregulation of the gut microbiota-host interaction is associated with the pathogenesis and progression of metabolic disorders. The metabolic and immunological roles of bile acids in human health have led to novel therapeutic approaches to manipulate the bile acid pool size, composition, and function by targeting one or multiple components of the microbiota-bile acid-bile acid receptor axis.

Metabolic impact of persistent organic pollutants on gut microbiota

Emerging evidence supports that exposure to persistent organic pollutants (POPs) can impact the interaction between the gut microbiota and host. Recent efforts have characterized the relationship between gut microbiota and environment pollutants suggesting additional research is needed to understand potential new avenues for toxicity. Here, we systematically examined the direct effects of POPs including 2,3,7,8-tetrachlorodibenzofuran (TCDF), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), and polychlorinated biphenyls (PCB-123 and PCB-156) on the microbiota using metatranscriptomics and NMR- and mass spectrometry-based metabolomics combined with flow cytometry and growth rate measurements (OD₆₀₀). This study demonstrated that (1) POPs directly and rapidly affect isolated cecal bacterial global metabolism that is associated with significant decreases in microbial metabolic activity; (2) significant changes in cecal bacterial gene expression related to tricarboxylic acid (TCA) cycle as well as carbon metabolism, carbon fixation, pyruvate metabolism, and protein export were observed following most POP exposure; (3) six individual bacterial species show variation in lipid metabolism in response to POP exposure; and (4) PCB-153 (non-coplanar)has a greater impact on bacteria than PCB-126 (coplanar) at the metabolic and transcriptional levels. These data provide new insights into the direct role of POPs on gut microbiota and begins to establish possible microbial toxicity endpoints which may help to inform risk assessment.

The microbiome modulating activity of bile acids

Bile acids are potent antibacterial compounds and play an important role in shaping the microbial ecology of the gut. Here, we combined flow cytometry, growth rate measurements (OD₆₀₀), and NMR- and mass spectrometry-based metabolomics to systematically profile the impact of bile acids on the microbiome using in vitro and in vivo models. This study confirmed that (1) unconjugated bile acids possess more potent antibacterial activity than conjugated bile acids; (2) Gram-positive bacteria are more sensitive to bile acids than Gram-negative bacteria; (3) some probiotic bacteria such as Lactobacillus and Bifidobacterium and 7α-dehydroxylating bacteria such as Clostridium scindens show bile acid resistance that is associated with activation of glycolysis. Moreover, we demonstrated that (4) as one of most hydrophobic bile acids, lithocholic acid (LCA) shows reduced toxicity to bacteria in the cecal microbiome in both in vivo and in vitro models; (5) bile acids directly and rapidly affect bacterial global metabolism including membrane damage, disrupted amino acid, nucleotide, and carbohydrate metabolism; and (6) in vivo, short-term exposure to bile acids significantly affected host metabolism via alterations of the bacterial community structure. This study systematically profiled interactions between bile acids and gut bacteria providing validation of previous observation and new insights into the interaction of bile acids with the microbiome and mechanisms related to bile acid tolerance.

Perfluorooctane sulfonate alters gut microbiota-host metabolic homeostasis in mice

Perfluorooctane sulfonate (PFOS) is a persistent environmental chemical whose biological effects are mediated by multiple mechanisms. Recent evidence suggests that the gut microbiome may be directly impacted by and/or alter the fate and effects of environmental chemicals in the host. Thus, the aim of this study was to determine whether PFOS influences the gut microbiome and its metabolism, and the host metabolome. Four groups of male C57BL/6 J mice were fed a diet with or without 0.003 %, 0.006 %, or 0.012 % PFOS, respectively. 16S rRNA gene sequencing, metabolomic, and molecular analyses were used to examine the gut microbiota of mice after dietary PFOS exposure. Dietary PFOS exposure caused a marked change in the gut microbiome compared to controls. Dietary PFOS also caused dose-dependent changes in hepatic metabolic pathways including those involved in lipid metabolism, oxidative stress, inflammation, TCA cycle, glucose, and amino acid metabolism. Changes in the metabolome correlated with changes in genes that regulate these pathways. Integrative analyses also demonstrated a strong correlation between the alterations in microbiota composition and host metabolic profiles induced by PFOS. Further, using isolated mouse cecal contents, PFOS exposure directly affected the gut microbiota metabolism. Results from these studies demonstrate that the molecular and biochemical changes induced by PFOS are mediated in part by the gut microbiome, which alters gene expression and the host metabolome in mice.

Metatranscriptomic Analysis of the Mouse Gut Microbiome Response to the Persistent Organic Pollutant 2,3,7,8-Tetrachlorodibenzofuran

Persistent organic pollutants (POPs) are important environmental chemicals and continued study of their mechanism of action remains a high priority. POPs, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), and polychlorinated biphenyls (PCBs), are widespread environmental contaminants that are agonists for the aryl hydrocarbon receptor (AHR). Activation of the AHR modulates the gut microbiome community structure and function, host immunity, and the host metabolome. In the current study, male C57BL6/J mice were exposed, via the diet, to 5 µg/kg body weight (BW) TCDF or 24 µg/kg BW of TCDF every day for 5 days. The functional and structural changes imparted by TCDF exposure to the gut microbiome and host metabolome were explored via 16S rRNA gene amplicon sequencing, metabolomics, and bacterial metatranscriptomics. Significant changes included increases in lipopolysaccharide (LPS) biosynthesis gene expression after exposure to 24 µg/kg BW of TCDF. Increases in LPS biosynthesis were confirmed with metabolomics and LPS assays using serum obtained from TCDF-treated mice. Significant increases in gene expression within aspartate and glutamate metabolism were noted after exposure to 24 µg/kg BW of TCDF. Together, these results suggest that after exposure to 24 µg/kg BW of TCDF, the gut microbiome increases the production of LPS and glutamate to promote localized gut inflammation, potentially using glutamate as a stress response.